Studies are being made with I131-fibrinogen in animals on the mechanisms maintaining plasma fibrinogen at normal levels and the chemical messengers that stimulate or depress rate of fibrinogen synthesis. Studies are being made with C14-bicarbonate on factors that alter the synthetic rate of albumin, fibrinogen and other plasma proteins. Fairly pure antithrombin III, alpha 1 antitrypsin and alpha 2 macroglobulin are being prepared and labelled with I131 in order to study their reactions in vivo with plasma proteases, particularly thrombin and plasmin, and to trace the removal of the reaction products from the blood and their distribution in tissues. Mathematical models of urea cycle enzyme metabolites and of interstitial proteins are being studied. BIBLIOGRAPHIC REFERENCES: Carlson, T., Fradl, D. and E.B. Reeve: Prostaglandin El (PGEl) stimulation of fibrinogen synthesis. Fed. Proc. 34, #3, Abstract #404, 1975. Reeve, E.B., W. Lew and T. Carlson: The arginine-C14 (C14-bicarbonate) method of measuring plasma protein synthetic rate and its simplication. In Plasma Protein Turnover, Eds. R. Bianchi, G. Mariani, A.S. McFarlane; Macmillan, London 1975.